With the expansion of high-tech industry, the ensuring of metal resources has become a serious problem. Metal elements including rare metals, such as rare earth elements, indium, niobium, and manganese, as well as platinum metals, copper, zinc, aluminum, etc., are called “critical metals”, which are extremely important key materials in various industries in Japan. However, these metal elements are not produced in Japan, and they are mostly dependent on imports. It is expected that the consumption of critical metals including rare metals will continue to significantly increase. However, their reserves are highly unevenly distributed in some areas, and also the price rapidly fluctuates depending on the social situation, etc. Therefore, the supply state is extremely unstable. The Ministry of Economy, Trade and Industry of Japan has had several discussions about the general strategy for the stable supply of rare metals since fiscal year 2008, and established the “Strategy for Ensuring Rare Metals” in 2009. In this strategy, together with the securing of foreign resources (exploration development), the development of alternate materials, and storage, the importance of recycling is mentioned. In addition, the utilization of so-called “urban mine”, a vast quantity of metal resources, has also been drawing great attention. The Ministry of Environment and the Ministry of Economy, Trade and Industry established the “Study Group on Collection and Appropriate Treatment of Rare Metals from Used Small Home Electronics” in fiscal year 2008, and discussions are ongoing to promote the recovery and cyclic use of rare metal elements. Further, plating washing wastewater or metal processing wastewater also contains slight amounts of valuable metals. Considering the social situation surrounding metal resources, recovery and recycling from such wastewater are also important issues. In the construction of such a recovery/recycling system for valuable metals, the development and establishment of an efficient technique for the adsorption and separation of valuable metals have been urgently needed.
Generally, for the removal and recovery of metals, methods such as aggregation, coprecipitation, solvent extraction, and granular adsorbing materials have been used. In consideration of the facilities, environmental impact, and also recycling, a method that uses a granular adsorbing material, such as an ion-exchange resin or a chelating resin, is effective. These adsorbing materials have been widely used for the removal and recovery of metals. In particular, chelating resins have higher affinity than ion-exchange resins and thus can be regarded as optimal adsorbing materials (Nonpatent Documents 1 to 4). A chelating resin is believed to be capable of adsorbing and recovering heavy metals in a solution having high concentrations of salts, which is difficult to do with an ion-exchange resin. Currently, chelating resins having various functional groups, such as an iminodiacetic acid group, a low-molecular-weight polyamine group, an aminophosphate group, an isothionium group, a dithiocarbamic acid group, and a glucamine group, are commercially available. Among them, a chelating resin having introduced thereinto an iminodiacetic acid group, which is applicable to the adsorption of a wide range of metals, has been mainly used. However, the iminodiacetic-acid-type chelating resin also captures alkaline earth elements, such as calcium, often contained in a large amount in a solution to be treated, thereby inhibiting the capture of the target element or reducing the separation efficiency. In addition, the power of the iminodiacetic-acid-type chelating resin to forma complex with a metal element is not so high. Accordingly, in actual use, it often happens that a high recovery rate is not obtained. If a chelating resin that does not capture alkaline earth elements, etc., and captures the metal element to be recovered selectively and reliably at high speed can be developed, a high-concentration recovery liquid (eluent) with less impurities can be obtained. This makes it possible to solve the problems related to the recovery rate, cost, purity increase, and the like in the valuable metal recovery process. In addition, the chelating resin after metal elution can be acid-cleaned and used again for adsorption, whereby the cost related to adsorption and recovery can also be reduced. However, as mentioned above, the iminodiacetic-acid-type chelating resin under the present circumstances has low selectivity and is susceptible to inhibition by coexisting elements, and thus is difficult to apply to the valuable metal recovery process, where a high-purity, high-concentration solution is required.
A chelating resin is a granular adsorbing material like activated carbon and ion-exchange resins. A water treatment technique using these granular adsorbing materials has already been established and is expected to be heavily used also in the future. However, because of its granular form, such a granular adsorbing material has to be packed in a specific can when used, and thus is sometimes difficult to apply to some conditions of use or some installation environments. In addition, the chelating resin has a low adsorption rate, and thus it is difficult to quickly treat a large amount of water. Therefore, in order for such a chelating resin to satisfy various requirements, as well as the improvement of metal adsorption characteristics, the diversification of the adsorbing material form also has to be considered.
In order to solve such problems, some methods for producing a fibrous chelating adsorbing material that can be easily processed into various forms and can meet various demands have been disclosed. Patent Document 1 discloses a method for introducing a chelating functional group into fibers using a chemical grafting method. Patent Document 2 and Patent Document 3 disclose a method for introducing a chelating functional group into fibers by radiation exposure using a radical formation/graft polymerization method. Patent Document 4 discloses a method for injecting a low-molecular-weight chelating agent into general-purpose fibers under high-temperature and high-pressure conditions. These chelating fibers have sufficient functions and a high adsorption rate, and thus it is expected that a quick treatment is possible. However, there are problems in production. In a chemical grafting method, the kind of graftable fiber is limited, and also the production process is complicated. A radiation grafting method is advantageous in that it can be applied to various fibers unlike the chemical grafting method. However, for the handling of radiation, the operation is performed in a specific environment, and thus it cannot be regarded as a simple and inexpensive production method. In addition, although a chelating agent injection/impregnation method is also advantageous in that various fibers can be used, because this is an impregnation method under high-temperature and high-pressure conditions, the general versatility is low.
Patent Document 5 discloses a method for producing a fibrous metal-adsorbing material using a blend spinning method. According to this method, a long-chain ligand (a metal-adsorbing compound having a long molecular chain) is subjected to wet blend spinning together with viscose, which allows for mass production at low cost using existing facilities. This fibrous metal-adsorbing material, as formed into a nonwoven fabric, shows metal adsorption capacity that depends on the amount of blend spinning. Therefore, metal-adsorbing materials in various forms can be produced (Patent Document 6). This production method is simple. At the same time, metal adsorption characteristics can also be diversified by changing the long-chain ligand to be mixed. In addition, the fibrous metal-adsorbing materials disclosed in Patent Documents 5 and 6 are not only superior in terms of production methods, but also characterized in that because of the use of a long-chain ligand, they have higher complex-forming ability together with higher element selectivity as compared with iminodiacetic-acid-type chelating resins. Further, because metal-adsorbing functional groups are present only on the fiber surface, they are also characterized in that as compared with a granular adsorbing material having metal-adsorbing functional groups also inside the pores, even when the dipping rate is increased, the metal-adsorbing ability does not decrease. However, generally, a solution to be treated, from which metals are recovered, is an acidic solution containing hydrochloric acid, sulfuric acid, nitric acid, etc. Rayon is decomposed when exposed to high acidity. Therefore, the fibrous metal-adsorbing materials using rayon as the matrix disclosed in Patent Document 5 and Patent Document 6 are limited in use under acidic conditions. In addition, rayon is decomposed also by microorganisms in the environment, etc. Therefore, there are problems in that they cannot withstand continuous use over a long period of time or several cycles of reuse.